There has been a continuing need for a method of preparing isocyanatoorganosilanes, including isocyanatoalkylsilanes, in high yields and purities from economical and relatively non-hazardous raw materials without generating significant quantities of hazardous by-products and waste materials. Heretofore, isocyanatoorganosilanes have been made in relatively low volumes by inefficient or costly processes.
For example, isocyanatoorganosilanes have been prepared by processes involving addition of hydrosilanes to unsaturated isocyanates, particularly allyl isocyanate, in the presence of a noble metal catalyst. Allyl isocyanate is a highly toxic raw material of limited commercial availability.
Processes are also known in the art wherein isocyanatoalkylsilanes are prepared from carbamatoalkylsilanes at a low temperature in the liquid phase, or from aminoalkylsilanes and highly toxic phosgene by various routes. All liquid phase processes disclosed thus far suffer from one or more disadvantages of low yield, slow kinetics, need for highly toxic raw materials, need for extensive work-up or purification often in the presence of higher levels of close-boiling contaminants, and substantial generation of by-products and waste materials.
A method for making 2-isocyanatoethoxysilanes by liquid phase thermal rearrangement of N-silyl-2-oxazolidinones has also been disclosed. The bonding of the isocyanatoalkyl group to silicon atoms in these molecules is through a hydrolyzable silicon-oxygen bond, and the silane moiety does not contain additional alkoxy groups as are present and often necessary in current commercially useful isocyanatoalkylsilanes.
The present invention surprisingly provides a method wherein isocyanatoorganosilanes are provided in high yields and purities with no need to use highly toxic phosgene or highly toxic allyl isocyanate, with no generation of highly corrosive hydrogen chloride as a by-product, with no need to use inert solvents as diluents, and with minimal formation of by-products, contaminants, and waste materials. Because the method of the present invention can be operated continuously with very short residence times, a relatively small reactor is capable of large throughputs with a correspondingly small capital investment.
The method of the present invention can also provide isocyanatoorganosilanes which have not been prepared by methods known to those skilled in the art, including isocyanatoorganosilanes in which the isocyanate groups are attached to silicon atoms through branched hydrocarbon groups. Such branched hydrocarbon groups respond to a need for isocyanatoorganosilanes having isocyanate groups with varying degrees of reactivity, which provide corresponding desirable variations in the performance properties of products incorporating said isocyanatoorganosilanes, including wet strength, flexibility, and oxidation resistance.
The method of the present invention can also provide isocyanatoorganosilanes wherein the silicon atom bearing the isocyanatoorgano group is further substituted by siloxy groups. These compounds combine the high surface activity of low molecular weight siloxanes with the high reactivity of the isocyanate group, and are useful in providing improved coatings, particularly for metallic substrates such as in automotive applications.